1. Field of the Invention
The present invention relates to a multi-core fiber.
2. Description of the Related Art
For a multi-core fiber including a plurality of high refractive index portions (cores) extending along a fiber axis, there is a demand that a skew between optical signals propagating through the cores (a time gap caused by passages through a plurality of paths) is small. The skew between the cores sometimes results from the length difference between the cores, and sometimes results from the structure difference between the cores. If the skew between the cores is large, a problem is caused in parallel optical transmission using a plurality of cores. An optical fiber ribbon is also sometimes required to reduce the skew between optical fibers. Inventions regarding this skew reduction are disclosed in Japanese Unexamined Patent Application Publication Nos. 10-39185 and 2003-21763. However, a method capable of more easily reducing the skew is required. Further, since the optical fiber ribbon has a large cross-sectional area, it is preferably improved for higher density.
A multi-core fiber is superior to an optical fiber ribbon in that the skew resulting from the core length difference is small. Also, the multi-core fiber is superior in density. However, it is demanded that not only the skew between the cores but also crosstalk between the cores is reduced in the multi-core fiber. Japanese Unexamined Patent Application Publication No. 2011-209702 and IEICE Electronics Express, Vol. 6, No. 2, pp. 98-103 (NPR 1) describe techniques intended to reduce the crosstalk between cores in a multi-core fiber. These references describe that it is effective to make the propagation constant different between the cores in order to reduce the crosstalk between the cores in the multi-core optical fiber.